In the course of treating and preparing subterranean wells for production, a well packer is run into the well on a work string or a production tubing. The purpose of the packer is to support production tubing and other completion equipment, such as a screen adjacent to a producing formation, and to seal the annulus between the outside of the production tubing and the inside of the well casing to block movement of fluids through the annulus past the packer location. The packer is provided with anchor slips having opposed camming surfaces which cooperate with complementary opposed wedging surfaces, whereby the anchor slips are radially extendible into gripping engagement against the well casing bore in response to relative axial movement of the wedging surfaces.
The packer also carries annular seal elements which are expandable radially into sealing engagement against the bore of the well casing in response to axial compression forces. Longitudinal movement of the packer components which set the anchor slips and the sealing elements may be produced either hydraulically or mechanically.
After the packer has been set and sealed against the well casing bore, it should maintain sealing engagement upon removal of the hydraulic or mechanical setting force. Moreover, it is essential that the packer remain locked in its set and sealed configuration while withstanding hydraulic pressures applied externally or internally from the formation and/or manipulation of the tubing string and service tools without unsetting the packer or interrupting the seal. This is made more difficult in deep wells in which the packer and its components are subjected to high downhole temperatures, for example, as high as 600 degrees F., and high downhole pressures, for example, 5,000 pounds per square inch ("psi"). Moreover, the packer should be able to withstand variation of externally applied hydraulic pressures at levels up to as much as 15,000 psi in both directions, and still be retrievable after exposure for long periods, for example, from 10 to 15 years or more. After such long periods of extended service under extreme pressure and temperature conditions, it is desirable that the packer be retrievable from the well, with the anchor slips and seal elements being retracted sufficiently to avoid seizure against well bore restrictions that are smaller than the retracted seal assembly, for example, at a makeup union, collar union, nipple or the like.
Currently, permanent packers are used for long-term placement in wells requiring the packer to withstand pressures as high as 15,000 psi at 600.degree. F. Conventional permanent packers are designed in such a way that they become permanently fixed to the casing wall and that helps in the sealing of the element package. However, permanent packers must be milled for removal. One of the major problems involved in removing a permanent packer is that its element package normally has large metal backup rings or shoes that bridge the gap between the packer and the casing and provide a support structure for the seal element to keep it from extruding out into the annulus. The problem with that arrangement is that the large metal backup shoes act like a set of slips and will not release from the casing wall.
Present retrievable high pressure packers use multiple C-ring backup shoes that are difficult to retract when attempting to retrieve the packer. A further limitation on the use of high pressure retrievable packers of conventional design, for example, single slip packers, is that if there is any slack in setting of the packer, or any subsequent movement of the packer, some of the compression force on the element package is relieved. This reduces the total compression force exerted on the seal elements between the mandrel and the casing, therefore permitting a leakage passage to develop across the seal package.
Further, it is common knowledge in designing currently used retrievable high pressure packers that a longer slip can be used to more evenly distribute the load into the casing. However, what generally occurs is that a slip will reach a length with a corresponding length of slip tooth contact, such that it becomes difficult or impossible to achieve initial slip tooth penetration into the casing wall when setting the packer. There becomes so much tooth length in contact with the casing that the setting slip load is insufficient to anchor the packer.
Another problem in high temperaure, high pressure packers of any type involves the slips damaging the casing. With the axial loads and pressure differential loads at the design limits, the total axial force on the packer slip is almost 500,000 pounds. Discounting friction, this load is multiplied to a radial force into the casing wall when divided by the tangent of the slip/wedge contact angle. Since the packer may be set inside uncemented casing, potential casing damage is a major concern.
With conventional segmented slips, the inherent three- or four-point loading of the casing wall will deform the casing into a predisposed slip pattern, and the fully loaded unsupported casing will deform into roughly a triangle or a square, etc., corresponding to the number of individual slips used. Nodes will appear on the casing outer diameter corresponding to each slip segment. This result is not desirable, as it will then become very difficult to land and properly set another packer after the first one is removed. Further, as the tubing in such wells is typcially made of an expensive corrosion resistant alloy, scratches and indentations are to be avoided, as they can act as stress risers or corrosion points.
Therefore, what is needed is a packer capable of safely deploying at its design limits in totally unsupported casing, without damaging the casing.
Another problem with high pressure retrievable packers is that they cannot withstand high tubing loads during production and stimulation operations.
Another problem with high pressure retrievable packers is that no matter how well designed, they can sometimes accidentally release.
Therefore, it is an object of the invention to provide a retrievable packer that can operate efficiently at pressure differentials of 15,000 psi and temperatures to 600.degree. F. without releasing.
It is further an object of this invention to provide a retrievable packer that has a slip design that allows longer slips to be effectively used.
It is further an object of this invention to provide a tighter element seal and a more dependable sealing system.
It is further an object of this invention to provide a retrievable packer that cannot be accidentally released.